+1 Isn't the definition of competitive related to how many people apply versus how many slots there are? If 90 people apply for 90 positions and those 90 people have an avg step score of 260, that doesn't necessarily make the specialty competitive.
To some degree, although self selection muddies the waters.
If a specialty has traditionally been unkind to people with Step I scores <240 (or <230, or whatever number you want to pick), people may choose not to attempt to match into that specialty with scores that do not meet that cutoff.
Additionally, there are "unwritten" requirements for some specialties, such as research (many subspecialty fields are terribly difficult to match into without some research background). Other factors like AOA status can be hugely influential as well; in fields like Derm, Plastics, and ENT, almost half of (or more than half of, in the case of Derm) successful matches are AOA. The specialties considered most competitive are typically the ones for which you really need to have it all: grades, board scores, research, AOA, LORs, etc.
People on SDN like to talk about how, say, EM is becoming "really competitive", based largely on the fact that it, you know, tends to fill 100% before SOAP, and people fail to match into it. So,[and I am prepared to be flamed by people here] even though as a middle-of-the-road EM applicant in 4th year you might think "boy this might be tough to match into), how "competitive" was it to get to that middle-of-the-road EM applicant status in the first place? Not easy, surely, as medical school is never easy; but it's not the same as someone who has set themselves up as a stellar (or even middle of the road) Plastics candidate.
What it really comes down to is that when you speak of "competitiveness" of a specialty, you can consider it two ways.
1. How difficult it is to match into the specialty as a 4th year student applying "against" all the other applicants in that specialty
2. How difficult it is to match into the specialty from day 1 of medical school.
This is not to suggest that everyone who does not go into Derm/Plastics/whatever chose to "settle" for other specialties (and indeed, many people have no interest whatsoever in the more competitive fields), but it's very difficult to parse out everyone's initial goals and compare them with the ultimate results.
Think of it as an intention-to-treat analysis (a weak but somewhat applicable analogy). If you ignore all the people who entered medical school but then never considered applying to [insert super competitive field of choice] due to perceived difficulty in doing so, it may change the interpretation of the data.
